In the past, perforating systems for use in completing or reworking wells have been run into wells on a pipe string or wire line and positioned and supported on a hanger. Alternatively, the perforating assemblies may be run into the well using a monobore completion string. Still another method for positioning the perforating assemblies entails running them into the well on a tubing string, connecting them to a wireline and lowering them to the desired position in the well, where they are anchored to the well casing. The wireline is typically detached and removed from the perforating assembly before the perforating operation.
Of these methods, it has been found most preferable to seat one or more perforating guns on a hanger or anchor that has been lowered and set in the casing at the desired depth. After the perforating guns are in position, the lowering equipment can be removed from the vicinity of the perforation, or from the well entirely. Thus the amount of unnecessary equipment in the vicinity of the perforation is minimized.
Conventional hangers, however, must be run into the well before any tubing string is emplaced because the hangers are typically too large to pass through a tubing string. If a tubing string is already in place in a well, as in the case of a well being reworked, it is difficult to position a hanger in the casing below the end of the tubing string without first removing the tubing string. Removal of the tubing string is undesirable, particularly in cases where the tubing string comprises expensive pipe and/or connections and it is preferred to keep the handling of the string to a minimum. In such cases, a wireline can be used to lower individual perforating guns through the tubing to the desired depth. The disadvantage to using a wireline is that each gun is fired separately, pressure and flow from the formation begin as soon as the first gun is fired, and the perforating operation is greatly prolonged. Alternatively, a through-tubing bridge plug can be used to support several perforating guns, but such a plug is not removable. Hence, a removable hanger that can be lowered through a tubing string and set below the tubing string is desired.
The hanger should be able to support several perforating guns, so that a desired length of pipe can be perforated simultaneously under preferred low-pressure conditions.
The hanger should also be self-centering in the well, with the centering means also being passable through the tubing string. If the hanger does not include a centralizer, it can be cocked or off-center in the casing, with the result that the hanger will not set properly, or may not set at all. Even if the hanger does set, other equipment, such as perforating guns, will not seat properly thereon if the hanger is cocked or off-center.
After perforation, the perforating guns can either be retrieved or dropped to the bottom of the well, depending on several factors. Hence, a hanger is also desired that can be easily operated under either circumstance, i.e., adapted either to maintain its position in the casing or to release itself from the casing and drop to the bottom upon perforation. Other objects and advantages of the invention will appear from the following description.